Is there a way to have Calendar.app display its week numbers using the standard ISO 8061 week definition?

The standard says a week starts on Monday, and the first Thursday of the year defines the first week. So at the end of 2015, the standard week of 2015-12-28 to 2016-01-03 is Week 53. In contrast, Calendar.app in Yosemite & Mountain Lion shows Week 1.

I'm wondering:

  • What definition of week is Apple using? Perhaps simply the First of January is always Week 1?
  • Is there any way to change this default behavior to show standard weeks?
  • 1
    OS X 11 (El Capitan) adds a new calendar in “Language & Region” system preferences: “ISO 8601”. I haven’t figured out yet how and where it actually differs from the default “Gregorian”.
    – Crissov
    Commented Oct 4, 2015 at 18:56
  • I’d also love to know if this is possible on iOS. As of v 15.6.1, at least on my device, it appears that only Gregorian, Japanese, and Buddhist calendars are available. Seems a little odd. See screenshot: i.imgur.com/Oan4r51.jpg
    – EJ Mak
    Commented Sep 24, 2022 at 15:03
  • @EJMak I suggest you post a Question on that matter. Commented Sep 24, 2022 at 15:32
  • 1
    @BasilBourque As a matter of fact, I just did: apple.stackexchange.com/q/448109/72448
    – EJ Mak
    Commented Sep 24, 2022 at 16:19

2 Answers 2


Calendar System

System Preferences in Yosemite 10.10.5 (and all later OS X releases, through El Capitan 10.11.3) lets you change the calendar system used across apps. The default for a United States install is Gregorian. To get behavior compliant with the ISO 8601 standard, change to ISO 8601.

Choose System Preferences > Language & Region > Calendar (popup menu) > ISO 8601.

Screenshot of System Preferences > Language & Region > Calendar (popup menu) > ISO 8601

Below are screen shots of Apple’s Calendar.app, before and after switching between Gregorian calendar and ISO 8601 calendar. I added the "Gregorian"/"ISO" labels.

Note the week numbers shown on the left. Weeks 52 & 1 changed to weeks 52 & 53. In a standard week, January 1st of 2016 is in Week 53. In ISO 8601 notation for weeks, that would be the week 2015-W53, and January 1st 2016 would be 2015-W53-5 with the weekday number appended.

For more info, see an entire Wikipedia article dedicated to ISO week date.

screenshot of December 2015 in Gregorian calendar

screenshot of December 2015 in standard ISO 8601 calendar

CAVEAT I do not know the ramifications of changing your Mac calendar system from Gregorian to ISO 8601. I searched support.apple.com but found no documentation.


Apple use the standard calendar "Gregorian calendar".

In contrast the:

ISO week-numbering year (also called ISO year informally) has 52 or 53 full weeks. That is 364 or 371 days instead of the usual 365 or 366 days. The extra week is referred to here as a leap week, although ISO 8601 does not use this term. Weeks start with Monday. The first week of a year is the week that contains the first Thursday of the year (and, hence, always contains 4 January). ISO week year numbering therefore slightly deviates from the Gregorian for some days close to 1 January.

Anyway, if you want ISO week number use this link to install it.

ISO cal

  • The link in the Answer refers to webcal.fi, where you can choose from many useful calendars and even tweak their parameters. Commented Jun 14, 2015 at 17:08
  • Sorry, I fixed it
    – Ruskes
    Commented Jun 14, 2015 at 17:11

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